Freeze concentration systems are commonly used for desalting and for food processing. The technique involves producing a slurry from a feedstream containing a solution of at least one dissolved substance. The slurry comprises a concentrated mother liquor and crystals of one (or more) of the substances in solution. The crystals are separated from the mother liquor in a wash column or other purifier. In some cases the concentrate is the desired product. In other cases, the crystals are the desired product. In either case the rapid efficient production of crystals is vital.
A vital and necessary part of a freeze concentration system is the freeze crystallizer which converts the feedstream to the aforementioned slurry. The freeze crystallizer must produce crystals efficiently and be uniformly distributed in the mother liquor.
A common form of crystallizer uses indirect heat transfer. That is to say the feedstream or slurry is separated from the refrigerant by a heat transfer surface. The crystals formed on the heat transfer surface must not be permitted to accumulate and must be removed as soon as possible after being formed.
Heretofore, motor-driven scrapers have been the mainstay of devices for cleaning deposits from heat transfer surfaces. Representative of such devices are the Scraped Surface Exchangers made by Vogt Products of Louisville, Ky, using doctor blades and auger-type scrapers. They are clumsy, complicated, and difficult to maintain. The reason for this is quite obvious, as doctor blades and auger-type scrapers require motors, chain drives, guard seals, and, of course, augers.
A so-called "Amertap" condenser utilizes nonrigid balls circulated in the condenser tubes. These devices are also quite complicated and represent that each tube receives a ball on the average of every 5 minutes.
Scrapers have been used in evaporators or other heat exchange apparatus to remove scale and other deposits from the walls of the heat exchangers. The scale and other deposits were then removed from the system. For purposes of this discussion, the term "scale" will be used to designate deposits from liquids which are ancillary and generally deleterious to the heating processes and generally to be avoided, if possible. The scale and other deposits were treated as waste products and were not distributed back into the fluid being treated and recirculated.
One such heat exchanger is described in U.S. Pat. No. 3,259,179 to J. M. Leach. In particular, the Leach apparatus is devised for a heat exchanger used in an evaporation converter to desalt sea or brackish water. Salt or other raw water is admitted through openings into tubes where it is evaporated.
Precipitates accumulate scale generally on the walls of the tubes. The accumulated scale reduces the rate of heat transfer through the walls of the tubes causing a deterioration of efficiency. In accordance with the Leach patent, to remove the accumulated deposit, the evaporation process is stopped, and a large piston pushes many scrapers a short distance until they reach outlet. The scrapers are then hydraulically forced across the internal surface of the tubes, thereby scraping off scale from the surface of the tubes. The scrapers are returned to their original position after the scale is flushed out and the evaporation process is started again.
Another method is shown in U.S. Pat. No. 3,406,741, also to J. M. Leach. A batch-type liquid treatment is carried out. Piston-like scrapers are moved through cylinders by the movement of the liquid being treated. After the treatment of a particular batch is completed, the treated liquid is removed, and another batch is supplied.
The concepts described and claimed herein are simple and require no special tube surface requirements. There are no mechanical drives. The construction is compact, and the concept lends itself to scaling to desired capacity. The system also functions independent of crystallizer orientation.
All of the foregoing have been made possible in crystallizers in contrast to other devices which produce an undesirable scale such as evaporators because of a fundamental difference between the mode of operation and the results obtained by the crystallizers and other devices.
In a crystallizer the desired product is the crystals. They are to be removed as quickly as possible. They are to be recycled through the crystallizer to encourage growth. In the case of ice crystals (the most frequent form of crystal encountered) research has shown that the ice does not adhere tenaciously and may be easily harvested from the crystallizer surface, provided that it is removed quickly, typically every ten seconds. Consequently, simple and far less rugged methods as described herein may be employed for removal.
Having observed and discovered the foregoing, a number of significant and novel objects are proposed.
It is an object of the invention to provide a freeze crystallizer subassembly which overcomes the limitations and disadvantages of such prior art devices.
It is another object of the invention to provide a freeze crystallization subassembly which places crystals formed on heat transfer walls into the slurry immediately after the crystals are formed to discourage the crystals from grouping into less treatable clumps.
It is yet another object of the invention to provide means for continuously scraping the surfaces of the heat transfer walls to insure maximum heat transfer through the walls.
It is yet another object of the invention to provide means for continuously recirculating scraper balls through a crystallizer.
It is still another object of the invention to provide heat transfer means utilizing rigid scraper balls.
It is yet another object of the invention to define a freeze crystallizer process wherein crystals are formed in a heat transfer surface and are continuously scraped and recirculated and generally processed for use in concentration systems.
It is a further object of the invention to provide a freeze crystallizer means using scrapers in each heat exchanger tube in combination with means for synchronizing the movement of the individual scrapers.
In accordance with the invention, a freeze crystallizer for producing a slurry containing a mother liquor and crystals from a feed solution of at least two substances having different freezing points comprises a reservoir for receiving said feed, for storing slurry, and for supplying slurry. In addition, a heat exchanger is included. The heat exchanger has a freezer compartment for circulating refrigerant and a slurry compartment in which the slurry is circulated. The freezer and slurry compartments are separated by heat transfer walls. A movable scraper means is situated within the slurry compartment. It is configured to transverse and scrape the heat transfer walls. Slurry circulating means interconnecting the reservoir and the slurry compartment for circulating slurry from said reservoir through the slurry compartment and back to the reservoir is also provided. The circulating slurry is programmed to reciprocate the scraper means in the slurry compartment to scrape the heat transfer walls.
Also in accordance with the invention is a process for producing a slurry of a mother liquor and a solute from a feed solution of at least two substances with different freezing points comprising the steps of supplying the feed solution to a slurry reservoir, removing slurry from the reservoir and circulating it through a heat exchanger where the slurry is separated from a refrigerant by heat transfer walls and then back to said reservoir. The circulating slurry is used to reciprocate a scraper immersed in the slurry to clean the heat transfer walls.